An escalator moves passengers from one level in a building to another. The main elements of the escalator are an electric motor, a plurality of steps, two handrails on either side of the steps, a transmission unit and a frame. The electric motor drives the steps and the handrails through the transmission unit. It is critical to the safety of the passengers that the steps and the handrails travel at the same speed.
Currently, two devices may determine the handrails' speed and stop the escalator if the speed of the steps differs a predetermined amount from the speed of the handrails. The first device utilizes an escalator with a junction box as the transmission unit. The junction box consists of a shaft and two sprockets spaced along the shaft. One sprocket receives power from the motor and the other transmits power to the handrail. The junction box transmits power through chains.
The first device has a multiplicity of targets, two disks, two photosensors and external circuitry. The disks are attached at spaced locations on the shaft. The targets line the edge of each of the disk. The photosensors are mounted on the junction box adjacent to the disks and detect the speed differential between the disks. The external circuitry calculates the speed of the handrail from this data and transmits this information to a controller.
This device is unreliable due to backlash. Play in the links of the chains cause inaccurate sensor readings.
The second prior art device utilizes an escalator with a drive wheel, a guide and a pulley assembly as the transmission unit. The drive wheel receives power from the motor. The guide supports the handrail. The pulley assembly forces the handrail into contact with the drive wheel, so that power transfers efficiently.
The second device has a roller and an inductive sensor. A bracket supports the roller and the sensor. The bracket bolts to the guide. The roller bolts to the bracket. A surface of the roller comes into frictional contact with the handrail. The roller offers no resistance to the friction force created by the contact, so the roller revolves about a bolt at a speed equal to that of the handrail. A spring attaches to the bracket and forces the roller into contact with the handrail. Two sensor targets are mounted on the roller face at predetermined circumferentially disposed locations.
The sensor is cylindrical. It is disposed axially through the bracket at a position which facilitates detection of the targets. A gap exists between the sensor and the roller. The sensor emits and receives pulse signals as it detects the targets. Through various electrical components, the signals' frequency is converted to indicate the speed of the roller. Recall, the roller speed is also that of the handrail. If the speed of the handrails falls outside of preset limits, the sensor sends a signal to a controller of the escalator to sound an alarm and to stop the steps and the handrails simultaneously.
During operative conditions the escalator experiences vibrations, changes in direction and uneven forces on the handrail, which may result when a passenger leans to heavily on one of the handrails. These events warp the handrail away from the guide and exert forces on the roller. These forces move the roller upward away from the handrail. However, the spring mounting of the roller forces it downward back into contact with the handrail.
This device has two main problems; reliability and alignment. The sensor detects targets that are attached to the roller and any irregular movement of the handrail jostles the roller due to insufficient contact surface between the roller and the handrail. This causes the speed measurements to fluctuate and as a result the escalator often cuts off unnecessarily. Another problem is the sensor requires recurrent adjustment of its axial position to maintain the gap between the sensor and the roller. The cylindrical sensor is disposed through the bracket. When the handrail jostles the roller, it also jostles the sensor, which moves the sensor into contact with the roller. This contact may have several consequences for the sensor. The contact may throw off the readings of the sensor until adjustment can be made, it may shut off the escalator unnecessarily or it may damage or destroy the sensor.
The prior art device is also expensive. The bracket requires detailed machining, so that it can be adapted to connect to the handrail guide and to support many of the components of the device.